2012 June 26

Why teach circuits to bioengineers?

I’ve been posting quite a bit on the design of the labs for the new applied circuits course we are designing for bioengineering (and other) students, but I’ve not yet clearly articulated why we require bioengineering students to take a circuits course. Without a clear justification, it could be just another arbitrary hoop for students to jump through—I’m sure many of the bioengineering majors see the current circuit theory class that way.

The justification for an electronics course for bioengineers can be summed up in one word: Sensors.

A sensor (in this context, anyway) is a device that converts some physical or chemical property of interest into an electrical parameter: a voltage, a current, a resistance, a capacitance, an inductance, … . The purpose of an electronic circuit is to convert this electrical parameter into a numeric value that we can record on a computer or in a lab notebook (which often means going through a voltage or time representation). There are, of course, other applications of electronics, but connecting to sensors is the one that matters for bioengineers.

It is very common in biological equipment these days to use a digital camera as a sensor, but the design of cameras and image processing software is outside the scope of this class. I think we want to concentrate on one-dimensional sensors that produce just a scalar value (or, more commonly, a value that varies as a function of time).

So in choosing labs, I want to focus on what sensors we are using and what processing we are doing in the electronics to get a numerical result at the end. Perhaps the course should be titled something like “Introductory circuits with applications to sensors” rather than what we were considering earlier, “Circuits with applications to bioengineering”.

Let’s go through the labs I’ve been thinking about so far and see how they fit with this theme:

Potentiometer-based goniometer for joint angle measurement.

Linear resistance-based measurement.

Useful for lead in to servo motors?

Useful for control and feedback in robotics.

Thermistor-based temperature measurement.

Resistance-based sensor.

Non-linear response curve.

Use of voltage divider to linearize curve and convert to voltage output.

Electret microphone.

Measuring current and voltage while varying series resistor to determine whether current source or resistance model is better.

Using series resistance (voltage divider) to convert current or resistance value to a voltage value, which can be viewed on scope.

Amplifying signal to make large enough to record with analog-to-digital converter.

Electrical field measurements in an electrophoresis gel. This one doesn’t fit the new theme—I think that there may be several reasons for discarding this lab.

Conductivity of saline solutions. Used for measuring ecosystems (like rivers).

Useful for talking about conductivity of bulk materials.

Requires thought about materials for electrodes.

Skin conductance meter.

Similar problems to conductivity of saline solution for measurement.

Need recording of slowly time-varying signal.

Do-it-yourself EKG

Amplification of very weak signals (though not as weak as nanopipettes and nanopores)

Can also be applied to other muscles (EMG) for control of robotics.

Electrical noise

May be too difficult to get good signals.

Optical blood pulse detector

Optical sensing very common in biosensors.

About the simplest application of light sensing.

Several different light sensors possible, with different circuits needed to convert to voltage.

Pulse oximeter

Expansion of optical pulse detector

Uses two different light sources alternately.

Gets into calibration questions.

Breathalyzer

Appears to be a resistance measurement.

Relies on self heating.

Should appeal to college students.

Capacitance touch sensor.

A different modality (not resistance, current, or voltage).

Conversion to frequency instead of voltage.

It looks like I have enough labs here, but I don’t have any direct chemical detection (other than salinity by conductance). Also, I suspect that the EKG may be too difficult to get working, and skin conductance may vary too slowly, so we may need some other time-varying signals.

[…] use in a sensor-focused circuits class? I listed a bunch that I’ve been thinking about in Why teach circuits to bioengineers?, but in this post I want to focus a bit more on what criteria we should use for deciding which […]